1.Field of the Invention
The present invention relates to a suspension controller for use in a vehicle, such as, an automobile. More particularly, the present invention pertains to a suspension controller for a vehicle which enables the attitude of the vehicle to be maintained in an appropriate state even when there are changes in accelerating or braking forces acting on the vehicle or changes in centrifugal forces that act on the vehicle while cornering.
2. Description of the Related Art
When a vehicle which is at rest is moved forward by application of the accelerator, inertia acts on the vehicle in a backward direction, so that the load acting on the suspension units for the rear wheels increases while the load acting on the suspension units for the front wheels decreases. As a result, the suspension units for the rear wheels are contracted, while the suspension units for the front wheels are expanded, thus causing the front portion of the vehicle to rise. This is known as the "squat phenomenon". When the accelerator is released to a certain degree after the vehicle speed has reached a desired level, a reactive jerk occurs. When braking force is applied to a moving vehicle, inertia acts on the vehicle in the forward direction, so that the load acting on the suspension units for the front wheels increases, while the load acting on the suspension units for the rear wheels decreases. As a result, the suspension units for the front wheels are contracted, while the suspension units for the rear wheels are expanded, thus causing the front part of the vehicle to dive, which is known as the "nose dive phenomenon". Additionally, when the vehicle comes to a stop, a reactive jerk occurs. These unfavorable movements of the vehicle, i.e., the squat and nose dive phenomena and the reactive jerks described, appear suddenly and strongly in proportion to the degree of acceleration or deceleration that is caused to act on the vehicle at a given moment and also in proportion to the degree of change in acceleration or deceleration from the level that had been acting on the vehicle at that given moment.
Generally speaking, when a vehicle is driven around a corner, centrifugal force resulting from the rotation of the vehicle acts outwardly from the center of rotation on the vehicle. As a result, the load acting on the outer suspension units which are disposed on the outer side of the turning circle of the vehicle increases, while the load acting on the inner side suspension units decreases. Accordingly, the outer side suspension units are contracted and the inner side suspension units are expanded, thus causing the vehicle to tilt and roll. In such circumstances, the vehicle body is subjected to a resultant force from centrifugal and gravitational forces, and since the vehicle is tilting, the occupant of the vehicle is subjected to composite acceleration .that acts in a direction which is considerably offset from the direction perpendicular to the floor surface of the vehicle body, thus making the occupant feel uncomfortable.
The following attempts have been made to cope with the above-described problems which have in the past been experienced. According to one of the solutions, a height sensor is employed to detect the degree of inclination of the vehicle body. Detection results are fed back to a controller which serves to control each suspension unit so that the vehicle body is returned to the normal horizontal state. In this type of control, however, when the acceleration acting on the vehicle suddenly changes and the vehicle body is suddenly tilted a substantial degree, the control cannot promptly respond to the change in attitude of the vehicle body, so that the tilting of the vehicle body cannot be effectively controlled.
As a method of controlling a vehicle during cornering, the following procedure has been attempted. In order to keep the floor surface of the vehicle perpendicular to the direction in which composite acceleration is acting on the vehicle body at any given moment, the degree of acceleration acting on the vehicle body is detected by means of an accelerometer, and the direction of the composite acceleration is calculated. Then each suspension unit is controlled so that the floor surface of the vehicle is perpendicular to the calculated direction. In this arrangement, any change in the attitude of the vehicle body which is actually occurring is detected and the detected change in attitude is fed back to a controller in order to control each suspension unit. The control of each suspension unit is practically effected by supplying or discharging the working oil to or from the unit so as to adjust the expansion or contraction position of the suspension unit. A certain period of time is therefore required for each suspension unit to be brought into a desired expansion/contraction position. Accordingly, this time delay disadvantageously hinders the control from satisfactorily responding to any kaleidoscopic change in posture of the vehicle body.